1. Field of the Invention
This invention relates generally to a method and apparatus for generating electric power from a photovoltaic array and, more particularly, to a method and apparatus that maximizes power output of a photovoltaic array during varying ambient weather conditions and delivers the generated electric power to a battery.
2. Prior Art
In a typical and common application of a photovoltaic array, the photovoltaic array is used to supply electric energy and electric power directly to a load. While this configuration is adequate for daylight operation, the photovoltaic array may cease to provide electric energy and electric power during periods of darkness or periods of reduced incident solar radiation. Therefore, many photovoltaic array configurations include a battery that is charged by the photovoltaic array during periods of incident solar radiation. The energy stored in the battery can then be used to supply electric energy and electric power to a load during periods of darkness or periods of reduced incident solar radiation. This in turn causes the battery to discharge. With this type of configuration, therefore, the photovoltaic array and the battery act together to keep the load continuously supplied with electric energy and electric power and the battery is alternately charged and discharged.
In order to most efficiently use the electrical power generated by a photovoltaic cell or photovoltaic array, it is desirable to maximize the power generated by the photovoltaic cell or photovoltaic array, despite varying weather conditions. Maximizing the power generated by a photovoltaic cell or photovoltaic array requires the determination of the optimal operating conditions for the photovoltaic cell or photovoltaic array for the given weather conditions, i.e., it is necessary to find the operating point on the voltage-versus-current curve for the photovoltaic cell or photovoltaic array that maximizes the power output from the photovoltaic cell or photovoltaic array. In addition, a generator system incorporating a photovoltaic cell or photovoltaic array must be able to determine optimal operating parameters for the generator system for varying temperatures and varying amounts of solar radiation incident on the photovoltaic cell or photovoltaic array.
Various circuits have been developed for the purpose of supplying electric energy generated by a photovoltaic cell or an array of photovoltaic cells to a battery or load. For example, U.S. Pat. No. 4,510,434, issued to Assbeck et al., U.S. Pat. No. 4,494,180, issued to Streater et al., and U.S. Pat. No. 4,404,472, issued to Steigerwald, disclose such circuits. In Assbeck et al., the automatic setting of the optimum operating point of a DC voltage source involves changing the duty cycle of a switch in a DC chopper to create variations in the measured voltage of the DC voltage source and changing the duty cycle accordingly until the optimal operating point is reached. The apparatus disclosed in Assbeck et al., however, requires the monitoring of both the electric voltage and current produced by the solar generator, which increases the complexity of the disclosed apparatus. In Streater et al., a load is matched to a source so that optimal operating conditions are obtained, requiring that the load remain electrically connected to the system at all times. The control disclosed in Steigerwald uses a DC-to-AC inverter or DC-to-DC converter to create a commanded electric current for a solar array which is compared to the resulting measured electric current from the solar array. The difference between the two signals controls the electric current and the power drawn from the solar array.
Baker, in U.S. Pat. No. 4,375,662, discloses a system wherein the DC power supplied by a photovoltaic cell, or an array of such cells, to a load is controlled by monitoring the slope of the photovoltaic cell output voltage vs. current characteristic and adjusting the current supplied by the photovoltaic cell to the load so that the slope is approximately unity. The slope is monitored by incrementally changing the load on the photovoltaic cell and determining whether the resulting change in current derived from the cell is above or below a reference value, indicative of the cell voltage. In response to the change in the monitored current being above the reference value, the slope of a voltage vs. current curve is greater than unity and the load is adjusted to decrease the current supplied by the photovoltaic cell to the load. Conversely, in response to the current being less than the reference value, the slope of the voltage vs. current curve is less than unity and the load is adjusted to increase the current supplied by the photovoltaic cell to the load.
Each of the disclosed systems also continuously search for the operating conditions that maximize the power output of the photovoltaic cell or photovoltaic array. Once the optimal operating conditions have been determined, the systems intentionally change the operating conditions to non-optimal operating conditions and restart the process of determining the optimal operating conditions. Therefore, the disclosed systems do not provide for extended or sustained operation at the optimal operating conditions once the optimal operating conditions have been determined, which causes a loss in efficiency of the systems and reduces the power delivered by the photovoltaic device to a battery or load. Consequently, in spite of the well-developed state of solar array and photovoltaic array technology, there is still a need for a peak power tracker for a photovoltaic array that is simple to construct, operates the photovoltaic array at peak power output for significant periods of time, and allows the electric energy and electric power produced by the photovoltaic array to be stored in a battery.
Muljadi et al., in U.S. Pat. No. 5,747,967, disclose a method and apparatus for maximizing the electric power output of a photovoltaic array connected to a battery. The voltage across the photovoltaic array is adjusted through a range of voltages to find the voltage across the photovoltaic array that maximizes the electric power generated by the photovoltaic array and then is held constant for a period of time. After the period of time has elapsed, the electric voltage across the photovoltaic array is again adjusted through a range of voltages and the process is repeated. The electric energy and the electric power generated by the photovoltaic array is delivered to the battery which stores the electric energy and the electric power for later delivery to a load.
Adams et al., in U.S. Pat. No. 6,205,656, disclose a method for attaching a photovoltaic cell or an array of such cells, to a PC board. Thus, while the method of Adams et al. for attaching a photovoltaic device to a circuit board is known, and circuits for optimizing the power transfer from such a photovoltaic device to a load such as a rechargeable battery are known, there remains a need for a rechargeable battery having a photovoltaic cell and recharging circuitry integral with the battery.
It is a first object of the present invention to provide a rechargeable battery having light responsive recharging means integral therewith.
It is a further object of the present invention to provide a rechargeable battery having light responsive recharging means integral therewith wherein the light responsive recharging means comprises a photovoltaic cell.
It is a yet a further object of the present invention to provide a rechargeable battery having light responsive recharging means operable for generating power in response to exposure to light and a power transfer optimization circuit integral therewith, the circuit being operable for optimizing power transfer from the recharging means to the rechargeable battery under a variety of lighting conditions.
The above objectives are met by a battery module comprising a rechargeable battery, a printed circuit board, a photovoltaic cell affixed to an outer surface of said printed circuit board and a rechargeable battery and a recharging circuit, wherein said photovoltaic cell, said printed circuit board and said recharging circuit are affixed to, and integral with, said rechargeable battery. The term xe2x80x9cintegralxe2x80x9d, as used herein to describe a particular construction, means that the members comprising the construction are affixed to one another and are inseparable from one another in normal use. The rechargeable battery may have two end-mounted power output terminals and two side-mounted recharging terminals, each of said recharging terminals being in electrical communication with one of said power output terminals.